I like how LisaRose came at this with a sample of something you could say. I was thinking about this on the way home after I read it earlier at work. Here's what I would say:
Mom, dad (not sure who is involved in the pressuring), I just want you to know that I know you're trying to encourage me and I appreciate it. However, the pressure is discouraging to me and makes me feel like Jehovah can't love me like I am right now. I know that discouragement is a tool of Satan to make people give up and I don't want to do that. So I have to ask you to stop talking to me about it. I want to get baptized when I'm ready and I feel like this is making it about me and you instead of me and Jehovah, so please just let me form that relationship at my own pace.
Anyway, that's the road I would take. First, acknowledge that you know they're trying to help. Then let them know that instead of helping it is actually hurting. Discouragement is a big thing in the organization that is linked with depression and that they are scared of because it takes people out of the organization. Your parents certainly wouldn't want to do that. Hopefully that would be enough to get them to back off.